1. Field of the Invention
The invention relates to the generation and detection of Terahertz (THz) radiation, and more specifically to the generation and detection of pulsed Terahertz (THz) radiation in nonlinear optical crystals pumped by ultrashort optical pulses.
2. Description of Related Art
In the past decade, the science and technology on electromagnetic radiation with a spectral content in the 0.1 to 10 THz range (Terahertz or THz radiation, 1 THz=1012 s−1) have attracted widespread interest and evolved into a useful tool for a number of applications (see Ref. [1] for a review). Many materials are transparent for electromagnetic radiation in the THz range, but unlike X-rays the THz radiation does not have an ionizing effect on the material due to the low energy of its photons. This makes it possible to apply THz radiation in different areas such as imaging of biological tissue and the measurement of fundamental solid-state processes in semiconductor physics.
For some applications, namely THz absorption spectroscopy or spectroscopic imaging, tunable narrow-band THz pulses with nanosecond duration may be favorable due to their better spectral resolution. Such systems for generating narrow-band THz radiation with a well defined THz frequency by means of difference frequency generation in a nonlinear optical crystal are known, e.g. from U.S. Pat. No. 7,054,339. A pair of fiber lasers generates single-frequency outputs at frequencies ω1 and ω2. The nonlinear interaction process in the nonlinear optical crystal generates THz radiation at =ω1−ω2 (difference frequency generation, DFG). These systems are complicated because they need two lasers that have to be synchronized and combined.
For some applications it is desirable to have shorter THz pulses, e.g. in the picosecond (10−12 s) or sub-picosecond range. Such broadband THz pulses on a picosecond time scale offer additional benefits that are unique to this technique. The excellence of such broadband THz pulses is the possibility of a phase-coherent detection technique that provides the inherent advantage of a time-resolution that may be as short as a few tens of femtoseconds (10−15 s).
Broadband THz pulses may be generated by several methods which all employ femtosecond laser pulses, namely using photoconductive switches, semiconductor surfaces, and optical rectification (OR) in nonlinear optical crystals. A pump pulse in the picosecond or sub-picosecond range contains frequencies in a frequency band Δω around the central frequency ω. Such an ultrashort pulse may produce broadband electromagnetic radiation at THz frequencies in a nonlinear optical material if certain phase matching and velocity matching conditions are fulfilled. This process is known as optical rectification.
A frequently used technique for the coherent detection of THz pulses is electro-optic (EO) sampling, a process that is based on the interaction of an optical pulse with the THz wave in a nonlinear material. A THz pulse traveling through the crystal is able to change the polarization of a co-propagating probe pulse in the optical range. This change of polarization is a measure for the electric field of the THz pulse and can be detected by appropriate means, e.g. a polarization beam splitter in combination with two photodetectors. EO sampling enables coherent detection of the THz pulse.
Among different approaches to generate and coherently detect THz pulses, which all require femtosecond lasers, those based on nonlinear optical effects (optical rectification (OR) and electro-optic (EO) sampling, respectively [2]) are advantageous since they use optical pulses at wavelengths outside the material's absorption range. Therefore the emitted THz field scales with both optical pulse energy and source crystal thickness up to the coherence length (see Eq. (2)), whereas the THz emission from processes that involve the excitation of free charge carriers (e.g. in photoconductive switches) is limited to the optical absorption length of the optical radiation; furthermore there is a risk of damaging the source through high optical power in the latter case.
Two prerequisites are given for a nonlinear optical material to be useful for THz applications, especially THz generation via OR and detection via EO sampling. First is a sufficient nonlinear optical susceptibility X(2) and electro-optic (EO) coefficient r. Second is velocity-matching between the optical and the THz pulse, i.e. the THz and the optical pulse have to propagate through the crystal with the same velocity. Velocity-matching is characterized by the coherence length lc; the latter ought to be at least the crystal thickness, typically 0.1 to 1 mm. Due to dispersion, lc is a function of both the optical wavelength λ and the THz frequency v. Hence, the material of choice depends on the desired range of v and the available laser source.
Velocity-matching with THz pulses is achieved e.g. within the inorganic semiconductor ZnTe (zinc telluride) when one uses laser pulses at a wavelength of, for example, 822 nm, i.e., within the tuning range of the widely used Ti:sapphire femtosecond lasers. This factor made ZnTe the material of choice for the generation of pulses with a broadband spectrum below a frequency of 3 THz. ZnTe has an electro-optic coefficient r=4 pm/V and good velocity-matching between optical pulses from Ti:Sapphire lasers also at λ<800 nm [3]. Ti:Sapphire lasers, however, are still very complex, require a given space and maintenance and are costly.
Among inorganic semiconductors, GaAs with an optimum velocity-matching wavelength of 1.4 μm [3] is the most promising candidate for generation and detection with “telecommunication” wavelengths (around 1.5 μm) and has been demonstrated as a source and detection material with 1.56 μm pulses from a fiber laser [4]. However, its electro-optic (EO) coefficient is about a factor of two lower than that of ZnTe.
Generally, organic nonlinear optical materials offer several advantages for THz applications, namely their high nonlinear optical susceptibilites, low dielectric constants, and the almost unlimited possibility to design molecules for a specific application [5]. These molecules can be incorporated in either organic crystals or polymers. Although polymers may be efficient emitters and detectors of THz radiation [6], they suffer from fast degradation and limited thickness; disadvantages that apply for organic crystals to a much lesser extent.
A known organic nonlinear optical material suited for the generation of THz pulses is the crystal DAST (4-N,N-dimethylamino-4′-N′-methyl stilbazolium tosylate). The EO coefficient of DAST (r111=47 pm/V at λ=1535 nm [7]) is more than an order of magnitude higher than that of ZnTe or GaAs. Velocity-matching between THz pulses and optical pulses with a wavelength λ around 1300 nm in DAST has been observed [8],[20]. Therefore, THz generation with a high conversion efficiency in DAST is possible. However, the coherent detection by EO sampling in DAST is problematic as DAST has a high birefringence and is thus not suited for EO sampling.
Ref. [20] proposes to employ another effect, namely the focusing of the probe beam (“Terahertz induced lensing”, TIL) by a spatial variation of the refractive index of the DAST crystal caused by a co-propagating THz pulse. The change in the beam profile can be detected, e.g. by measuring the intensity at a certain location of a screen onto which the optical probe beam is directed. The detection via TIL, however, may have a reduced sensitivity as compared to EO sampling in ZnTe if the relevant electro-optical coefficients are the same.
It is, therefore, an objective of the invention to provide a system and a method for broadband THz generation and detection that has an increased conversion efficiency in the generation step and an increased sensitivity in the detection step.
It is a further objective of the invention to provide a system and a method for broadband THz generation and detection that has a reduced complexity, does not require permanent maintenance and attention and is thus suited for commercial use, e.g. as a THz spectrometer and/or imaging device.